1. Field of the Invention
The invention relates to a ball screw device, a linear actuator and a vehicle steering system.
2. Description of Related Art
For example, some of electric power steerings that apply steering assist force to a rack shaft include a ball screw device (for example, see Japanese Patent Application Publication No. 2000-225956 (JP-A-2000-225956), FIG. 6 to FIG. 7C). The ball screw device includes a nut that is rotated by an electric motor. A threaded shaft is inserted through the nut. Balls are arranged between the nut and the threaded shaft. As the nut rotates, the threaded shaft linearly moves in the axial direction. As a result, a tie rod coupled to the threaded shaft is displaced to pivot a knuckle arm to thereby change the direction of a wheel.
In addition, the nut is provided with a circulating member for circulating the balls in the nut. A tube or a bridge is used as the circulating member (for example, see Japanese Patent Application Publication No. 2004-36789 (JP-A-2004-36789), [0016] and FIG. 1, Japanese Patent Application Publication No. 5-306741 (JP-A-5-306741), and FIG. 6, Japanese Utility Model Application Publication No. 4-107551 (JP-U-4-107551), FIG. 6, Japanese Patent Application Publication No. 2001-122138 (JP-A-2001-122138), [0002] and FIG. 5A to FIG. 5C, Japanese Patent Application Publication No. 2007-85502 (JP-A-2007-85502), [0011], FIG. 1 and FIG. 2, and the pamphlet of International Publication No. WO2007/126084, [0074] and FIG. 16).
When the tube is used as the circulating member, the tube projects from the ball nut by a large amount, and the ball screw device radially increases in size. On the other hand, when the bridge formed of a small piece is used as the circulating member, the ball nut may be radially reduced in size. In the case where the bridge is used, a hole is formed in the ball nut, and the bridge is inserted in the hole. In addition, in the ball screw device, or the like, of the electric power steering, a large radial load acts on the ball nut from a road surface via the wheel, the rack shaft, and the like. Therefore, as is described in the pamphlet of International Publication No. WO2007/126084, [0074] and FIG. 16, it is desirable that an annular flange fixed to the outer periphery of the ball nut is used to couple the ball nut to a rotor of the electric motor.
Specifically, the ball nut and the rotor are arranged coaxially with each other, and the annular flange formed on the rotor is matched with the annular flange of the ball nut. Then, bolts are inserted through insertion holes formed in the annular flanges to fix the annular flanges. The plurality of bolts are arranged at equal intervals in the circumferential direction of the annular flanges. When the bridges are fitted to the ball nut as described above and the flanges are fixed around the outer periphery of the ball nut, it is possible to achieve reduction in the size of the ball nut in the radial direction and high load bearing performance. However, in the pamphlet of International Publication No. WO2007/126084, [0074] and FIG. 16, the bridges and the annular flange of the ball nut are arranged so as to deviate from each other in the axial direction, so the ball nut increases in size in the axial direction.